Discoloration resistant cellulose article and method of manufacture



United States Patent 3,256,110 DISCOLORATION RESISTANT CELLULOSE ARTI- CLE AND METHOD OF MANUFACTURE George A. Crowe, Jr., Plainfield, N.J., assignor, by mesne assignments, to FMC Corporation, Marcus Hook, Pa.,

a corporation of Delaware No Drawing. Filed July 1, 1964, Ser. No. 379,719

' 4 Claims. (Cl. 117-56) This application is a continuationdn-part of my application Serial No. 157,568 filed December 6, 1961, and now abandoned.

The present invention relates to regenerated cellulose fibers and theinmanufacture, and more particularly to the stabilization of regenerated cellulose fibers and articles made therefrom against degradation on storage or exposure to high levels of energy.

While this invention will be described with reference to fibers which include continuous rayon filaments as well as rayon staple fibers, it should be understood that it is equally applicable to films, ribbons, bands, casings, and the like, and the term article as used above and hereinafter is intended to be inclusive.

Regenerated cellulose articles have an inherent tendency to become discolored and lose tensile strength on storage, particularly when they have been exposed to high levels of energy. This presents a serious problem with light colored articles which must be stored or used over a prolonged period of time, particularly when they must be exposed to heat,- ionizing radiation or other forms of radiant energy in use, .hot air drying and sterilization, for example. Discoloration of regenerated cellulose articles is a particularly vexing problem in surgical articles such as surgical dressings, sponges, wipes and the like since it is highly desirable that such articles be white to connote cleanliness and lack of impurities, and the usual sterilization techniques employing steam, or high energy radiation from an electron beam, Roentgen ray machine, cobalt 60 or the like almost invariably cause substantial yellowing of the rayon. Substantial discoloration, i.e. yellowing of the article, is always extremely objectionable in a normally white or colorless surgical dressing.

Various methods of reducing the discoloration of cellulosic materials have been proposed. G. H. White, Jr., for example, has suggested in United States Patent 2,922,728, issued January 26, 1960, that this be accomplished by treating the cellulose with a small amount of tartaric, citric or gluconic acid. The present invention provides improved methods for the stabilization of refilaments or staple to remove soluble impurities there- 3,256,110 Patented June 14, 1966 from and as a separate and final liquid treatment or as part of a conventional finish bath. After the application,

the excess aqueous solution is removed and the filaments or staple are then dried in the presence of the stabilizing agents.

The. total amount of calcium nitrate and citric acid or citrate ions which is employed may be varied within certain specific limits. It has been found, in general, that a useful degree of improvement in the stability of the regenerated cellulose articles is obtained by treatment with an aqueous solution containing at least about 0.025 percent by weight of each of the stabilizing agents in order to obtain a highly satisfactory degree of stabilization. The preferred concentration of each of the components in the calcium nitrate-citric acid bath is about 0.05 to 0.10 percent by weight. The stabilizing solution is, of course, on the acid side, and preferably within the range of from 4.0 to 5.0.

As heretofore mentioned, the stabilization procedure is preferably carried out as a separate and final liquid treatment on the completely processed and washed but never-dried regenerated cellulose filament or fiber. In

, this process the rayon article is immersed or otherwise removed from the rayon article as by wringing or any generated cellulose articles against discoloration and loss effect which would be expected from the combined effects 7 of the individual components of the combination.

It is convenient to combine the stabilizing agents calcium nitrate and citric acid and into one aqueous solution which is applied to the regenerated cellulose articles while they are still in a never-dried condition or after they have been dried. Preferably, the aqueous solution containing the calcium nitrate and citric acid is applied within the normal operations for manufacturing viscose filaments or staple after washing of such never-dried other suitable means, after which the treated article is dried in air or a vacuum by conventional procedures to remove liquid water and leave the treating agents in situ on the article. It has been found that the rayon article normally picks up about its own weight of treating solution although it may retain as much as 1.5 to 2 times its own weight of solution depending upon the wringing technique or other method used to remove the excess solution. It is apparent, therefore, that when the treated article is dried it will normally retain about the same concentration of treating agents by weight of the rayon as was in the treating solution by weight of the solution although in some cases the concentration on the regenerated cellulose may be as much as 1.5 to 2 times the concentration in the treating solution.

The invention will now be described in greater detail in the following specific examples which are illustrative only and are not to be construed as limiting the invention.

EXAMPLE I Samples of never-dried bleached rayon which had been completely processed and washed free of soluble impurities were immersed in aqueous treating solutions containing various concentrations of citric acid alone and various concentrations of calcium nitrate alone as well as in aqueous solutions containing both citric acid and calcium nitrate in various concentrations. All of the samples were allowed to remain in contact with the treating solutions until thoroughly wetted after which the excess treating solution was removed by wringing and the damp samples dried in air. The treated samples were then conditioned by storage at 70 to 72 F. at a relative humidity of 65 percent until equilibrium was achieved. The samples were then sterilized by exposure to 2.5 megarads of cobalt 60 radiation and aged by storage at F. for 28 days. The relative stability of the treated samples was then determined by comparison of their colors with unstabilized control samples and other stabilized materials.

Standard test procedures The relative degree of discoloration of rayon samples is determined according to the ASTM standard test procedure designation D1365-55T which employs a color difference meter available from the Gardner Laboratory,

Bethesda, Maryland. The primary standard for reflectance measurement is a layer of magnesium oxide prepared according to the instructions of the National Bureau of Standards. Although other color characteristics may be compared by the standard test, the chief coordinate of interest in the present case is coordinate b which indicates specimen colors from yellow to blue; minus values of b being blue, positive values, yellow and 0.0 being the value obtained for the White primary standard described above. Because of the differences between instruments and the difiiculties of making a primary standard, porcelain enamel plaques are calibrated against the primary standard and used as working standards. These plaques are available from the National Bureau of Standards, Washington DC. and the Gardner Laboratories, Bethesda, Maryland. All values used for comparison of the relative stability of the rayon samples referred to herein are expressed in plus scalar values of the b coordinate obtained by the standard test procedure which indicates the degree of yellowing of the sample.

The plus b values obtained when the samples were tested on the color difference meter according to the standard test procedure are set out in the table below.

These plus b values afford a basis for comparing the degree of yellowing and, therefore, the relative degree of stabilization of the various samples. As noted above a reading of b=0.0 indicates the color of the standard white sample whereas a plus value of b indicates yellowing; the greater the plus value of b the greater the yellowing and consequently the lower the degree of stabilization against discoloration.

STABILITY OF RAYON STERILIZED BY EXPOSURE T0 2.5 ME GARADS OF COBALT 60 RADIATION AND AGED AT 140 F. FOR 28 DAYS The data in the above table show that treatment of rayon according to the preferred embodiment of the invention in which calcium nitrate and citric acid are employed in combination, provides a greater degree of stabilization than can be achieved by either calcium nitrate or citric acid when used alone and a greater degree of stabilization than would be expected from the combination based on a knowledge of the effects obtained by the individual components.

It is apparent from the foregoing data that the present invention is capable of substantially improving the stability of regenerated cellulose articles even when exposed to very high levels of energy such as 2.5 megarads of cobalt 60 radiation. The invention is also useful for stabilizing rayon when exposed to lower energy levels such as those encountered in steam sterilization. It has been found that very low concentrations of calcium nitrate and citric acid, for example concentrations of from "0.002 percent to 0.01 percent by weight of each of the stabilizing agents, incorporated in an aqueous finishing bath are capable of stabilizing rayon against discoloration during and subsequent to sterilization with steam.

The stabilizing agents of the present invention are operable under a wide variety of conditions. They may be employed to stabilize rayon against degradation by exposure to energy levels such as provided by dry heat at temperatures of the order of 250 F. or the moist heat of steam sterilization at temperatures of 240 to 260 F. for periods of 20 to 45 minutes. The present invention employing calcium nitrate in combination with citric acid provides a method which is unique in that it is capable of stabilizing rayon against degradation on exposure to higher levels of energy such as ionizing radiation. The invention is capable of providing substantial stabilization of rayon whether exposed to mild doses of low energy infrared radiation or large doses of ionizing radiation up to about 5 megarads or higher. Although it is gen erally preferred to employ the stabilizing agents in a ratio of 1:1 by weight, large ratios of calcium nitrate to citric acid of the order of 30:1 or more may be employed. However, inasmuch as excess citric acid may have deleterious elfect on rayon it is preferred to employ ratios of citric acid to calcium nitrate of no more than about The stabilization system of the present invention has many advantages over previously available systems, particularly for surgical purposes. Inasmuch as calcium nitrate and citric acid are fully oxidized they do not react with stainless steel hospital equipment such as sterilization vessels, hypodermic syringes and the like, which is a disadvantage of the bisulfite stabilization systems of the prior art. This is also an advantage in that the new stabilizing system does not reduce iodine or similar medicaments or antiseptic solutions and do not bleach dyes used in such preparations.

Unless stated otherwise all percentages, concentrations and ratios recited herein are on a weight by weight basis. It will be obvious to those skilled in the art that the invention is susceptible of many modifications within its spirit and accordingly it is to be limited only by the scope of the appended claims.

I claim:

1. A method of manufacturing a discoloration resistant cellulose article comprising washing a preshaped regenerated cellulose article free of soluble impurities, thoroughly wetting the article with a solution containing as color stabilizing agents from about 0.05 percent .to about 0.20 percent by weight of each calcium nitrate and citric acid and drying the article in the presence of the stabilizing agents. 1

2. Method of manufacturing a discoloration resistant cellulose fiber comprising washing fully regenerated and never-dried fibers of viscose rayon free of soluble impurities, thoroughly wetting the fibers with a solution containing as color stabilizing agents about 0.05 percent to about 0.20 percent by weight of each of calcium nitrate and citric acid and drying the fibers in the presence of the stabilizing agents.

' 3. A method as defined in claim 2 wherein an amount of said solution contains from about 0.05 percent to 0.10 percent of each calcium nitrate and citric acid. I

4. An article of manufacture prepared accordingto the method of claim 1.

References Cited by the Examiner UNITED STATES PATENTS 2,922,728 1/1960 White 117114 XR 3,012,844 12/1961 Screnock 117-144 X WILLIAM D. MARTIN, Primary Examiner. H. E. COLE, W. D. HERRICK, Assistant Examiners. 

1. A METHOD OF MANUFACTURING A DISCOLORATION RESISTANT CELLULOSE ARTICLE COMPRISING WASHING A PRESHAPED REGENERATED CELLULOSE ARTICLE FREE OF SOLUBLE IMPURITIES, THROUGHLY WETTING THE ARTICLE WITH A SOLUTION CONTAINING AS COLOR STABILIZING AGENTS FROM ABOUT 0.05 PERCENT TO ABOUT 0.20 PERCENT BY WEIGHT OF EACH CALCIUM NITRATE AND CITRIC ACID AND DRYING THE ARTICLE IN THE PRESENCE OF THE STABILIZING AGENTS. 